1. Field of the Invention
This invention relates to a method for making a positive temperature coefficient (PTC) device, more particularly to a method for making a PTC device that includes crosslinking a crosslinkable preform after soldering a pair of conductive leads to a pair of electrodes on the crosslinkable preform.
2. Description of the Related Art
A PTC composite material consisting of polymer and electrical conductive filler exhibits a PTC property such that the resistance of the PTC composite material is increased exponentially when the temperature thereof is raised to its melting point. Hence, the PTC composite material is commonly used as a fuse, such as a thermistor, for protecting a circuit from being damaged.
Referring to FIG. 1, a conventional method for making a PTC device 1 includes consecutive steps of: (A) sheeting a blend 11 of a PTC composition; (B) attaching a pair of electrodes 12 to the blend 11 of the PTC composition so as to sandwich the blend 11 of the PTC composition therebetween; (C) irradiating the blend 11 of the PTC composition so as to crosslink the same using an irradiating apparatus 17; and (D) soldering a pair of conductive leads 13 to the electrodes 12 using a lead-free solder paste 14 in a reflow oven 15 so as to form the PTC device 1.
Since the reflow oven 15 is required to be operated at a temperature sufficient to melt the lead-free solder paste 14 for the soldering operation, which is relatively high, undesired breaking of hydrogen bonds of the molecular structure of the crosslinked blend 11 of the PTC composition is likely to occur, which, in turn, results in a deviation from the specification in the resistance requirement for the products of the PTC device 1 and a reduction of the production yield.
In addition, the way of heating during the soldering of the leads 13 to the electrodes 12 in the reflow oven 15, i.e., by heating the upper one of the leads 13 through a heated gas blown from above and the lower one of the leads 13 through a metallic support 151 of the reflow oven 15 that is in contact therewith, can cause a non-uniform temperature distribution throughout the PTC device. As a consequence, when the PTC device is cooled down, the cooling rate throughout the crosslinked blend 11 of the PTC composition will be uneven, which results in an increase in the resistance of the crosslinked blend 11 of the PTC composition, which, in turn, results in an increase in power consumption during the use of the PTC device 1.